A voltage-controlled oscillator or VCO is an electronic oscillator with its oscillation frequency controlled by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency or phase modulations.
A frequency divider is an electronic circuit that takes an input signal with an input frequency and generates an output signal with an output frequency equal to the input frequency divided by some integer n.
VCOs having multiple output phases form key building blocks for many wired and wireless communication systems. Frequency dividers are widely used in devices such as frequency synthesizers and as signal generators.
FIG. 1 is a circuit diagram of a conventional VCO 10. The VCO 10 includes a pair of NMOS transistors M1 and M2 that are cross-coupled to provide amplification. Elements T1 and T2 represent two segments of a transmission line to provide a half lambda (λ/2) delay line. The VCO 10 also includes two varactors C1 and C2 whose capacitances are tuned by the applied voltage Vtune. For low oscillation frequencies, T1 and T2 may be replaced by inductors. This circuit topology is simple and provides good performance for a high-speed oscillator. However, the VCO 10 cannot provide more than two-phased oscillations, and its circuit topology cannot be adapted to provide a VCO with an odd number of phases.
FIG. 2 shows that the basic topology shown in FIG. 1 can be adapted to provide a divide-by-two frequency divider 20. With comparison to the VCO 10 of FIG. 1, the varactors C1 and C2 are removed and a common NMOS pulldown transistor M3 is disposed between the source terminals of the transistors M1 and M2 and the ground node. The input signal Vin drives the gate terminal of the pulldown transistor M3. If the transmission line setup is nearly half of the input frequency, the output Vo+ and Vo− will oscillate at half of the input frequency. This kind of frequency divider is referred to as an “injection locked frequency divider.” Like VCO 10, the frequency divider 20 cannot provide more than two-phased oscillations.
While multi-phase VCOs exist in the art, these VCOs have proved difficult to design and make. The ad hoc design solutions have tight design constraints and their topologies are not easily adapted to designs having different phase numbers. In short, it has proved difficult to design a simple, high-performance VCO architecture that is generally applicable to any selected number of phases. The same difficulties can be found with the frequency dividers of the prior art.